arXiv subjects

Sambaran Banerjee

Publications and source records attributed to Sambaran Banerjee.

At least 19 recordsLinked to original sources

arXiv2026

Examining the stellar-merger origin of the blue main sequence in the open cluster NGC\,3532 with N-body simulations

Extended main-sequence turnoffs, extended main sequences, and split main sequences observed in the colour-magnitude diagrams of young and intermediate-age star clusters are now widely interpreted as the consequence of a distribution of stellar rotation rates among their intermediate-mass (1.5-1.8~M$_\odot$) members. However, the origin of the slowly rotating population that occupies the blue main sequence (bMS) remains uncertain, and stellar mergers have been proposed as one possible pathway. We investigate whether stellar mergers can account for the observed bMS population in the 330-Myr Galactic open cluster NGC 3532. We perform fourteen direct NBODY7 simulations spanning different initial cluster masses, radii, binary fractions, and binary orbital distributions. The simulations are selected to reproduce the present-day properties of NGC 3532 within the observational uncertainties, allowing us to estimate the expected number of merger products among its intermediate-mass main-sequence (MS) members. The cluster hosts $\approx 37\%$ bMS members among its intermediate-mass MS population, which are predominantly slow rotators. Despite this large bMS population, our simulations produce only a handful of MS-MS merger products, due to the cluster's low density and substantial mass loss during its evolution. Stellar mergers are unlikely to be the dominant formation channel for the observed slowly rotating bMS population in NGC 3532 and other disperse open clusters. Our results instead favour angular-momentum loss mechanisms operating before or shortly after the zero-age main sequence, such as pre-main-sequence star-disk interactions or tidal synchronization in low-mass ratio binaries.

astro-ph.GA↗

arXiv2026

Stellar-mass black holes in young massive and open stellar clusters -- VII. Comparisons with gravitational-wave events until LVK-O4a and Gaia compact binaries

Gravitational-wave (GW) detections by the LIGO-Virgo-KAGRA (LVK) observatories suggest multiple formation channels for GW compact binary mergers. Here I assess the role of young massive clusters (YMC) evolving into old open clusters (OC) - the YMC/OC channel - to the GW merger population. A homogeneous grid of 90 N-body evolutionary model star clusters, spanning initial masses of $10^4M_\odot\leq M_{cl}(0)\leq10^5M_\odot$, half-mass radii of 1-3 pc, and metallicities between 0.0002-0.02 is computed with the direct, post-Newtonian N-body code NBODY7. The N-body simulations include primordial binaries, delayed stellar-remnant model forming black holes (BH) and neutron stars (NS), BH spin prescriptions, and GW recoil kicks, and they are evolved until BH depletion. Most GW mergers from the cluster models are dynamically assembled binary black holes (BBH) that merge within their host clusters. Merger mass ratios reach 0.1-0.2 despite an overall bias toward nearly symmetric pairs. The GW merger efficiency varies non-monotonically with cluster mass, peaking around $M_{cl}(0)=7.5\times10^4M_\odot$ and also for $M_{cl}(0)\leq3.0\times10^4M_\odot$. The computed mergers reproduce some of the key features of the latest observed GW event catalogue, including asymmetric low-mass mergers, misaligned events among highly spinning, massive BHs, and an excess of $30M_\odot$ primaries, though they under-produce $10M_\odot$ primaries, hinting at contributions from other channels. The model merger rate density accounts for a fiducial 25%-33% of the observed rate; it increases with redshift somewhat faster than the cosmic star formation, consistently with LVK's inferences. The model effective spin distribution is positively asymmetric at zero redshift and broadens with redshift. The models yield field BH- and NS-main sequence star binaries with parameters consistent with the Gaia-discovered candidates. [Abgd]

astro-ph.GA↗

arXiv2025

The velocity dispersion profile of nine open clusters in the solar neighborhood

We analyze the velocity dispersion profiles of nine open clusters in the solar neighborhood using kinematic data from Gaia DR 3, aiming to identify potential dynamical signatures of stellar-mass black holes through a comparison of theoretical and observed dispersion profiles. The selected clusters include LP2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. We refine the center positions of the clusters with the Meanshift algorithm. Using the Markov Chain Monte Carlo method, we calculate the velocity dispersion for each cluster and construct one-dimensional velocity dispersion profiles. NGC 2516, NGC 3532, and NGC 6475 show potential central cusps in their radial velocity dispersion profiles, which may indicate the presence of stellar-mass black holes. LP2373 gp4, NGC 6475, and Praesepe all display a negative correlation between velocity dispersion and stellar mass, indicating these clusters are approaching energy equipartition or expanding. NGC 2516 and NGC 3532 exhibit a positive dependence between velocity dispersion and stellar mass, which may be attributed to the preferential ejection of massive stars following dynamical interactions involving binaries or black holes. These two clusters are the only two that are dynamical not relaxed and are closest to virial equilibrium. We compare the observations with N-body simulations of star clusters. A comparison of observed and simulated velocity dispersion profiles reveals that NGC 2516 and NGC 3532 exhibit lower proper motion dispersions than model clusters. Better agreement with the observed profiles is achieved for model clusters with larger ages. This suggests that the observed clusters may have undergone rapid dynamical evolution. Our results suggest that NGC 2516 and NGC 3532 may host at least two stellar-mass black holes each.

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arXiv2025

Stellar-mass black holes in young massive and open stellar clusters -- VI. Role of external galactic field

Young massive clusters (YMC) and open clusters (OC) are widely considered as potential environments for assembling merging binary stellar-remnant black holes (BBH) via dynamical interactions. However, such moderate mass systems are susceptible to being disrupted by the external tidal field of their host galaxies, potentially limiting their effectiveness as GW sources. In this study, I investigate the formation of BBH mergers in tidally dissolving star clusters. This is achieved with a newly computed grid consisting of 95 evolutionary model star clusters, where the clusters are subjected to a varying extent of tidal stripping. The cluster evolutions are computed with the direct N-body integrator NBODY7 that includes, among others, treatments for post-Newtonian (PN) effects in compact-binary members, mass loss due to stellar evolution, formation of stellar remnants, and tidal stripping. It is found that even strong tidal stripping does not quench the formation of a black hole (BH) core inside a cluster or the formation of dynamical BBH mergers in the system. The overall properties of BBH mergers, e.g., the form of the distribution of merger delay time, primary mass, and mass ratio, and the redshift evolution of merger rate are not significantly altered by the extent of tidal stripping of the parent cluster population. Furthermore, even strongly tidally stripped clusters are capable of dynamically forming Gaia-BH-like detached BH--main-sequence-star binaries that escape into the galactic field. Limitations of the present study and potential future improvements are discussed.

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arXiv2025

X-ray emission from helium star+black hole binaries as probes of tidally induced spin-up of second-born black holes

Tidally induced spin-up of stripped helium stars in short-period (<\,1\,d) binaries with black holes has been presented as one of the possible mechanisms to reproduce the high-spin tail of the black hole spin distribution derived from gravitational wave (GW) merger observations. At such short periods, a fraction of the strong stellar wind from the stripped helium stars may be accreted by the black holes, and its gravitational potential energy may be released as observable radiation in the X-ray regime. We estimate the X-ray luminosity and its observability from black holes in orbit with stripped helium stars, which evolve into binary black hole or black hole+neutron star binaries that merge within Hubble time. We post-process recent advancements for estimating X-ray luminosities (via wind accretion onto stellar mass black holes) into two rapid population synthesis codes, BSE and StarTrack. We derive lower limits on the X-ray luminosity distribution from populations of stripped helium+black hole binaries at four metallicities (0.01, 0.1, 0.5, 1 $Z_{\odot}$) and two mass transfer stability criteria. We find that a large fraction (0.1-0.5) of stripped-helium stars that get spun up by tides also transfer enough wind matter onto the black hole to produce X-ray luminosities above $10^{35}$\,erg\,s$^{-1}$, up to $\sim10^{39}$\,erg\,s$^{-1}$. Such binaries should be observable as X-ray bright systems at 0.1\,$Z_{\odot}$, 0.5\,$Z_{\odot}$ and $Z_{\odot}$, representative of Sextans A, the Large Magellanic Cloud (LMC) and the Solar neighbourhood, respectively. The formation efficiency of these systems increases with decreasing metallicity. However, accounting for the local star formation rates, our population synthesis predicts $\sim$2 and $\sim$1 such observable binaries in the Milky Way and LMC, respectively, that will produce a binary compact object merger within a Hubble time. (Abridged)

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arXiv2024

On the effective spin-mass ratio relation of binary black hole mergers that evolved in isolation

The ground-based measurement of gravitational waves (GW) from merging binary black holes (BBH) allows independent determination of spins of stellar-remnant black holes (BH). The observed population of BBH mergers exhibits two intriguing peculiarities related to BH spins, namely, a positively biased distribution of the effective spin parameter, $\chi_{\rm eff}$, and an apparent anti-correlation between merger mass ratio, $q$, and $\chi_{\rm eff}$. Here we investigate the potential mechanisms for such observed properties, in BBH mergers via isolated binary evolution. We synthesise BBH mergers with the fast binary evolution code BSE. The role of various physical assumptions is explored, including tidal spin-up, compact remnant mass, and mass transfer physics. We compare the properties of BBHs that form through stable mass transfer (SMT) and common envelope evolution (CE). We find that both the asymmetry in the $\chi_{\rm eff}$ distribution and the $\chi_{\rm eff}$ anti-correlation can be natural outcomes of isolated-binary BBH formation. The anti-correlation is especially pronounced for SMT-channel BBH mergers that experience a mass-ratio reversal, i.e., those where the second-born BH is the more massive one. The anti-correlation arises from the dependence of orbital shrinking during mass transfer and the Roche lobe size on the system's mass ratio. This characteristic $\chi_{\rm eff}-q$ trend diminishes with increasing metallicity and when the isolated-binary BBH merger population is mixed with a significant contribution of dynamically formed BBH mergers or the newly formed BH's spin is misaligned relative to the parent star's spin. Our results demonstrate that isolated massive binary evolution via the SMT sub-channel can reproduce trends in the observable BBH merger population, with the characteristic signatures in mass, mass ratio, and spin distributions.

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arXiv2024

The enigmatic origin of two dormant BH binaries: Gaia BH1 and Gaia BH2

The two systems, namely, Gaia~BH1 and Gaia~BH2, that have been confirmed as dormant (i.e., no X-ray emission detected) black hole (BH) - low-mass star binaries in the latest Gaia mission data release (DR3) are intriguing in the context of their formation and evolution. Both systems consist of $\sim9$ $\mathrm{M}_{\odot}$ BH and $\sim1$ $\mathrm{M}_{\odot}$ star orbiting each other on a wide, eccentric orbit ($e\sim 0.5$). We argue that formation of such Gaia~BH-like systems through the isolated binary evolution (IBE) channel, under the standard common envelope assumptions, and from dynamical interactions in young massive and open clusters are equally probable, and that the formation rate of such binaries is of the order of $10^{-7}$ $\mathrm{M}_{\odot}^{-1}$ for both channels. We estimate that, according to our models, there are at most $\sim900$ detectable Gaia~BH-like binaries in the Milky Way thin disc. What plays an important role in formation of Gaia~BH-like systems via the IBE channel is the mutual position of the natal kick velocity vector and the binary angular momentum vector. We find that natal kicks with a median magnitude of $\sim40$ km/s are preferred for the formation of Gaia~BH1-like binaries. Approximately $94\%$ of those binaries are formed with the BH spin misaligned to the orbital axis by less than $40^{\circ}$. Gaia~BH2-like binaries form if the low velocity natal kick (of median magnitude $\sim20$ km/s) is directed within $15^{\circ}$ about the orbital plane. In addition to natal kick, we also discuss the influence of tidal interaction and the adopted common envelope $\lambda_\mathrm{ce}$~parameter prescription on the evolution of Gaia~BH-like binaries. We follow the subsequent evolution of the binaries, once formed as Gaia~BH1 and Gaia~BH2 systems, to investigate their connection with the low-mass X-ray binary population.

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arXiv2023

Symmetry breaking in merging binary black holes from young massive clusters and isolated binaries

Properties of the to-date-observed binary black hole (BBH) merger events suggest a preference towards spin-orbit aligned mergers. Naturally, this has caused widespread interest and speculations regrading implications on various merger formation channels. Here we show that (i) not only the BBH-merger population from isolated binaries, but also (ii) BBH population formed in young massive clusters (YMC) would possess an asymmetry in favour of aligned mergers, in the distribution of the events' effective spin parameter ($χ_{\rm eff}$). In our analysis, we utilize BBH-merger outcomes from state-of-the-art N-body evolutionary models of YMCs and isolated binary population synthesis. We incorporate, for the first time in such an analysis, misalignments due to both natal kicks and dynamical encounters. The YMC $χ_{\rm eff}$ distribution has a mean (an anti-aligned merger fraction) of $\langleχ_{\rm eff}\rangle\leq0.04$ ($f_X-\approx40\%$), which is smaller (larger) than but consistent with the observed asymmetry of $\langleχ_{\rm eff}\rangle\approx0.06$ ($f_X-\approx28\%$) as obtained from the population analysis by the LIGO-Virgo-KAGRA collaboration. In contrast, isolated binaries alone tend to produce a much stronger asymmetry; for the tested physical models, $\langleχ_{\rm eff}\rangle\approx0.25$ and $f_X-\lesssim7\%$. Although the YMC $χ_{\rm eff}$ distribution is more similar to the observed counterpart, none of the channels correctly reproduce the observed distribution. Our results suggest that further extensive model explorations for both isolated-binary and dynamical channels as well as better observational constraints are necessary to understand the physics of 'the symmetry breaking' of the BBH-merger population.

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arXiv2022

Binary black hole mergers from young massive clusters in the pair-instability supernova mass gap

The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of $\approx100M_\odot$, and as well as other massive BBH merger events involving BHs within the pair-instability supernova (PSN) mass gap have sparked widespread debate on the origin of such extreme gravitational-wave (GW) events. In this study, I investigate whether dynamical interactions in young massive clusters (YMCs) serves as a viable scenario for assembling PSN-gap BBH mergers. To that end, I explore a grid of 40 new evolutionary models of a representative YMC of initial mass $M_{\rm cl}=7.5\times10^4M_\odot$ ($N\approx1.28\times10^5$) and size $r_h=2$ pc, with all BH progenitor stars being initially in primordial binaries. All cluster models are evolved with the direct, relativistic N-body code NBODY7 incorporating up to date remnant formation, BH natal spin, and general-relativistic (GR) merger recoil schemes. The BBH mergers from these model cluster computations agree well with the masses and effective spin parameters of the GW events in the latest GW transient catalogue (GWTC). In particular, GW190521-like, i.e., $\approx200M_\odot$, low aligned spin events are produced via dynamical merger among BHs derived from star-star merger products. GW190403-like, i.e., PSN-gap, highly asymmetric and aligned events result from mergers involving BHs that are spun up via matter accretion or binary interaction. The present YMC models yield a present day, intrinsic merger rate density of $0-3.8\times10^{-2}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ for GW190521-type events. They produce GW190403-like events at a rate within $0-1.6\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$ and their total BBH-merger yield within the PSN gap is $0-8.4\times10^{-1}{{\rm~yr}^{-1}{\rm Gpc}^{-3}}$.

astro-ph.HE↗

arXiv2021

Merger rate density of stellar-mass binary black holes from young massive clusters, open clusters, and isolated binaries: comparisons with LIGO-Virgo-KAGRA results

I investigate the roles of cluster dynamics and massive binary evolution in producing stellar-remnant binary black hole (BBH) mergers over the cosmic time. To that end, dynamical BBH mergers are obtained from long-term direct N-body evolutionary models of $\sim10^4M_\odot$, pc-scale young massive clusters (YMC) evolving into moderate-mass open clusters (OC). Fast evolutionary models of massive isolated binaries (IB) yield BBHs from binary evolution. Population synthesis in a Model Universe is then performed, taking into account observed cosmic star-formation and enrichment histories, to obtain BBH-merger yields from these two channels observable at the present day and over cosmic time. The merging BBH populations from the two channels are combined by applying a proof-of-concept Bayesian regression chain, taking into account observed differential intrinsic BBH merger rate densities from the second gravitational-wave transient catalogue (GWTC-2). The analysis estimates an OB-star binary fraction of $f_{\rm Obin}\gtrsim90$% and a YMC formation efficiency of $f_{\rm YMC}\sim10^{-2}$, being consistent with recent optical observations and large scale structure formation simulations. The corresponding combined Model Universe present-day, differential intrinsic BBH merger rate density and the cosmic evolution of BBH merger rate density both agree well with those from GWTC-2. The analysis also suggests that despite significant 'dynamical mixing' at low redshifts, BBH mergers at high redshifts ($z_{\rm event}\gtrsim1$) could still be predominantly determined by binary-evolution physics. Caveats in the present approach and future improvements are discussed.

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arXiv2021

Black hole mergers in compact star clusters and massive black hole formation beyond the mass-gap

We present direct N-body simulations, carried out with Nbody6++GPU, of young and compact low metallicity star clusters with $1.1\times 10^5$ stars, a velocity dispersion of $\sim$ 10 $\mathrm{km\,s^{-1}}$, a half mass radius $R_h=0.6$ pc, and a binary fraction of $10\%$ including updated evolution models for stellar winds and pair-instability supernovae (PISNe). Within the first tens of megayears of evolution, each cluster hosts several black hole (BH) merger events which nearly cover the complete mass range of primary and secondary BH masses for current LIGO/Virgo/Kagra gravitational wave detections. The importance of gravitational recoil is estimated statistically. We present several possible formation paths of massive BHs above the assumed lower PISNe mass-gap limit ($45 M_\odot$) into the intermediate-mass BH (IMBH) regime ($> 100 M_\odot$) which include collisions of stars and BHs as well as the direct collapse of stellar merger remnants with low mass cores. The stellar evolution updates result in the early formation of higher mass stellar BHs than for the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star-BH mergers, we present a first-generation formation scenario for GW190521-like events, a merger of two BHs in the PISN mass-gap, which is dominated by star-BH mergers. This IMBH formation path is independent of gravitational recoil and therefore conceivable in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153$M_\odot$,173$M_\odot$) which is expected to merge within a Hubble time.

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arXiv2021

Binary black hole mergers from young massive and open clusters: comparison to GWTC-2 gravitational wave data

Several astrophysical scenarios have been proposed to explain the origin of the population of binary black hole (BBH) mergers detected in gravitational waves (GWs) by the LIGO/Virgo Collaboration. Among them, BBH mergers assembled dynamically in young massive and open clusters have been shown to produce merger rate densities consistent with LIGO/Virgo estimated rates. We use the results of a suite of direct, high-precision $N$-body evolutionary models of young massive and open clusters and build the population of BBH mergers, by accounting for both a cosmologically-motivated model for the formation of young massive and open clusters and the detection probability of LIGO/Virgo. We show that our models produce dynamically-paired BBH mergers that are well consistent with the observed masses, mass ratios, effective spin parameters, and final spins of the second Gravitational Wave Transient Catalog (GWTC-2).

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arXiv2021

Stellar-mass black holes in young massive and open stellar clusters V: comparisons with LIGO-Virgo merger rate densities

I study the contribution of young massive star clusters (YMCs) and open star clusters (OCs) to the present-day intrinsic merger rate density of dynamically-assembled binary black holes (BBHs). The BBH merger event rate is estimated based on a set of 65 state-of-the-art evolutionary models of star clusters, as presented in Banerjee (2021). These relativistic direct many-body computed models incorporate up-to-date stellar mass loss and remnant formation ingredients. The merger-event rates are obtained by constructing a cluster population of the Universe, out of the models, taking into account mass distribution of clusters and cosmic star formation and enrichment histories, as per observations. The model BBH merger rate density ranges from a pessimistic to a reference value of $0.5{\rm~yr}^{-1}{\rm Gpc}^{-3}-37.9{\rm~yr}^{-1}{\rm Gpc}^{-3}$, for a LIGO-Virgo-like detector horizon. The reference rate well accommodates the BBH merger rate densities estimated from GWTC-1 and GWTC-2 merger-event catalogues. The computed models also yield differential BBH merger rate densities that agree reasonably with those from GWTC-1 and, as well, with the much more constrained ones from GWTC-2. These results suggest that dynamical interactions in YMCs and OCs can, in principle, alone explain the BBH merger rate density and its dependence on the merging-binary properties, as inferred from to-date gravitational-wave (GW) events. The cosmic evolution of merger rate density from the computed models is also studied. The models predict a rate of $\approx5{\rm~yr}^{-1}{\rm Gpc}^{-3}$ for eccentric LIGO-Virgo mergers from YMCs and OCs. The improving constraints on BBH merger rate density with mounting GW events will help constraining scenarios of star cluster formation across cosmic time and as well the relative contributions of the various compact binary merger channels.

astro-ph.HE↗

arXiv2020

LISA sources from young massive and open stellar clusters

I study the potential role of young massive (YMCs) and open star clusters (OCs) in assembling stellar-mass binary black holes (BBHs) which would be detectable as persistent gravitational-wave (GW) sources by the forthcoming LISA mission. The energetic dynamical interactions inside star clusters make them factories of assembling BBHs and other types of double-compact binaries that undergo general-relativistic (GR) inspiral and merger. The initial phase of such inspirals would, typically, sweep through the LISA GW band. Here, such LISA sources are studied from a set of evolutionary models of star clusters with masses ranging over $10^4M_\odot-10^5M_\odot$ that represent YMCs and intermediate-aged OCs in metal-rich and metal-poor environments of the Local Universe. These models are evolved with long-term, direct, relativistic many-body computations incorporating state-of-the-art stellar-evolutionary and remnant-formation models. Based on models of Local Universe constructed with such model clusters, it is shown that YMCs and intermediate-aged OCs would yield several 10s to 100s of LISA BBH sources at the current cosmic epoch with GW frequency within $10^{-3}{\rm~Hz} - 10^{-1}{\rm~Hz}$ and signal-to-noise-ratio (S/N) $>5$, assuming a mission lifetime of 5 or 10 years. Such LISA BBHs would have a bimodal distribution in total mass, be generally eccentric ($\lesssim0.7$), and typically have similar component masses although mass-asymmetric systems are possible. Intrinsically, there would be 1000s of present-day, LISA-detectable BBHs from YMCs and OCs. That way, YMCs and OCs would provide a significant and the dominant contribution to the stellar-mass BBH population detectable by LISA. A small fraction, $<5$%, of these BBHs would undergo GR inspiral to make it to LIGO-Virgo GW frequency band and merge, within the mission timespan; $<15$% would do so within twice the timespan.

astro-ph.HE↗

arXiv2020

Demographics of neutron stars in young massive and open clusters

Star clusters appear to be the ideal environment for the assembly of neutron star-neutron star (NS-NS) and black hole-neutron star (BH-NS) binaries. These binaries are among the most interesting astrophysical objects, being potential sources of gravitational waves (GWs) and gamma-ray bursts. We use for the first time high-precision N-body simulations of young massive and open clusters to study the origin and dynamical evolution of NSs, within clusters with different initial masses, metallicities, primordial binary fractions, and prescriptions for the compact object natal kicks at birth. We find that the radial profile of NSs is shaped by the BH content of the cluster, which partially quenches the NS segregation due to the BH-burning process. This leaves most of the NSs out of the densest cluster regions, where NS-NS and BH-NS binaries could potentially form. Due to a large velocity kick that they receive at birth, most of the NSs escape the host clusters, with the bulk of their retained population made up of NSs of $\sim 1.3$ M$_\odot$ coming from the electron-capture supernova process. The details of the primordial binary fraction and pairing can smear out this trend. Finally, we find that a subset of our models produce NS-NS mergers, leading to a rate of $\sim 0.01$--$0.1$ Gpc$^{-3}$ yr$^{-1}$ in the local Universe, and compute an upper limit of $\sim 3\times 10^{-2}$--$3\times 10^{-3}$ Gpc$^{-3}$ yr$^{-1}$ for the BH-NS merger rate. Our estimates are several orders of magnitude smaller than the current empirical merger rate from LIGO/Virgo, in agreement with the recent rate estimates for old globular clusters.

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arXiv2020

Formation of low-spinning 100 Msun black holes

It is speculated that a merger of two massive stellar-origin BHs in a dense stellar environment may lead to the formation of a massive BH in the pair-instability mass gap (50-135 Msun). Such a merger-formed BH is expected to typically have a high spin (a=0.7). If such a massive BH acquires another BH it may lead to another merger detectable by LIGO/Virgo in gravitational waves. Acquiring a companion may be hindered by gravitational-wave kick/recoil, which accompanies the first merger and may quickly remove the massive BH from its parent globular or nuclear cluster. We test whether it is possible for a massive merger-formed BH in the pair-instability gap to be retained in its parent cluster and have low spin. Such a BH would be indistinguishable from a primordial BH. We employed results from numerical relativity calculations of black hole mergers to explore the range of gravitational-wave recoil velocities for various combinations of merging BH masses and spins. We compared merger-formed massive BH speeds with typical escape velocities from globular and nuclear clusters. We show that a globular cluster is highly unlikely to form and retain a 100 Msun BH if the spin of the BH is low (a<0.3) as such BHs acquire high recoil speeds (>200 km/s) that exceed typical escape speeds from globular clusters (50 km/s). However, a very low-spinning (a=0.1) and massive (100 Msun) BH could be formed and retained in a galactic nuclear star cluster. Even though such massive merger-formed BHs with such low spins acquire high speeds during formation (400 km/s), they may avoid ejection since massive nuclear clusters have high escape velocities (300-500 km/s). A future detection of a massive BH in the pair-instability mass gap with low spin would therefore not be proof of the existence of primordial BHs, which are sometimes claimed to have low spins and arbitrarily high masses.

astro-ph.HE↗

arXiv2020

Stellar-mass black holes in young massive and open stellar clusters IV: updated stellar-evolutionary and black hole spin models and comparisons with the LIGO-Virgo O1/O2 merger-event data

I present a set of long-term, direct, relativistic many-body computations of model dense stellar clusters with up-to-date stellar-evolutionary, supernova (SN), and remnant natal-kick models, including pair instability and pulsation pair instability supernova (PSN and PPSN), using an updated version of NBODY7 N-body simulation program. The N-body model also includes stellar evolution-based natal spins of BHs and treatments of binary black hole (BBH) mergers based on numerical relativity. These, for the first time in a direct N-body simulation, allow for second-generation BBH mergers. The set of 65 evolutionary models have initial masses $10^4M_\odot-10^5M_\odot$, sizes 1 pc-3 pc, metallicity $0.0001-0.02$, with the massive stars in primordial binaries and they represent young massive clusters (YMC) and moderately massive open clusters (OC). Such models produce dynamically-paired BBH mergers that agree well with the observed masses, mass ratios, effective spin parameters, and final spins of the LVC O1/O2 merger events, provided BHs are born with low or no spin but spin up after undergoing a BBH merger or matter accretion onto it. In particular, the distinctly higher mass, effective spin parameter, and final spin of GW170729 merger event is naturally reproduced, as also the mass asymmetry of the O3 event GW190412. The computed models produce massive, $\sim100M_\odot$ BBH mergers with primary mass within the 'PSN gap' and also yield mergers involving remnants in the 'mass gap'. They also suggest that YMCs and OCs produce persistent, Local-Universe GW sources detectable by LISA. Such clusters are also capable of producing eccentric LIGO-Virgo mergers.

astro-ph.HE↗

arXiv2020

BSE versus StarTrack: implementations of new wind, remnant-formation, and natal-kick schemes in NBODY7 and their astrophysical consequences

The masses of stellar-remnant black holes (BH), as a result of their formation via massive single- and binary-stellar evolution, is of high interest in this era of gravitational-wave detection from binary black hole (BBH) and binary neutron star (BNS) mergers. Here we present new developments in the N-body evolution program NBODY7 in regards to its stellar-remnant formation and related schemes. We demonstrate that the newly-implemented stellar-wind and remnant-formation schemes in the NBODY7 code's BSE sector, such as the 'rapid' and the 'delayed' supernova (SN) schemes along with an implementation of pulsational-pair-instability and pair-instability supernova (PPSN/PSN), now produces neutron star (NS) and BH masses that agree nearly perfectly, over large ranges of zero-age-main sequence (ZAMS) mass and metallicity, with those from the StarTrack population-synthesis program. We also demonstrate the new implementations of various natal-kick mechanisms on NSs and BHs such as the 'convection-asymmetry-driven', 'collapse-asymmetry-driven', and 'neutrino-emission-driven' kicks, in addition to a fully consistent implementation of the standard, fallback-dependent, momentum-conserving natal kick. We find that the SN material fallback causes the convection-asymmetry kick to effectively retain similar number and mass of BHs in clusters as for the standard, momentum-conserving kick. The collapse-asymmetry kick would cause nearly all BHs to retain in clusters irrespective of remnant formation model and metallicity, whereas the inference of a large number of BHs in GCs would potentially rule out the neutrino-driven kick mechanism. Pre-SN mergers of massive primordial binaries would cause BH masses to deviate from the single-star ZAMS mass-remnant mass relation. Such mergers, at low metallicities, can produce low-spinning BHs within the PSN mass gap that can be retained in a stellar cluster.

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